This invention relates to high density recording of digital data on a movable magnetic medium and particularly to such recording on a magnetic tape where some variation of speed in its movement is inevitable.
Various codes have been employed in efforts to achieve high density recording on magnetic media. Some of these codes and their general defining parameters, F(X, Y) where X is the minimum number of possible locations passed in order to encounter a flux change and Y is the maximum, are:
F(3, 12) for 3PM (three position modulation) PA1 F(1, .infin.) for NRZI (nonreturn to zero) PA1 F(1, 2) for MPE (Manchester phase encoded) PA1 F(2, 4) for MFM PA1 F(2, 5) for 6250 type group encoding PA1 F(1, 17) for ENRZ
While 3PM achieves high bit efficiency, three bits per two flux changes, the application to tape systems generally produces unsatisfactory error rates because of the three position windows for each bit cell, or cell position, combined with tape speed variation. On high performance disc systems the 3PM code may be satisfactory. The present invention combines some of the advantages of the above codes without the disadvantages and at the same time has more tolerance of speed variations. The NRZI code is desirable in that the allowable time window for flux change occurrence encompasses one complete bit cell; however, the maximum number of possible cell positions, or locations, between flux changes is unbounded. MFM overcomes this by inserting an extra flux change between recurring zeros, but in doing so divides the observation window by two. MFM, 3PM and other codes require read recovery systems with sampling error corrections to the timing recovery. With 3PM this time base sampling may be as many as twelve possible locations apart.
Enhanced NRZ or ENRZ which is NRZI with a parity bit every ninth location has been used with some success; however, the maximum number of possible locations between flux changes is sixteen. The present invention employs a new code having similar efficiency, one bit per flux change, to ENRZ with eight bits of data resulting from nine bits of code. This provides eight bits per nine flux changes. However, ENRZ has a possible sixteen windows with no flux change where the code employed in the present invention has a maximum of nine. The code employed in the present invention also provides a recognizable character framing bit in that the ninth bit is always a flux change. Error checking codes are provided by block cyclic redundancy check (CRC) characters. The present invention employs the above described code hereinafter referred to as NRZJ in combination with a unique character at the end of the preamble and at the beginning of the postamble to a block of data to achieve a record suitable for bi-directional reading, suitable for providing a clock signal with no more than nine locations therebetween and suitable for providing recognition of the end of a data block immediately upon the reading of the unique character.
It is therefore an object of the present invention to provide an improved system for arranging digital data for high density recording on a movable magnetic medium.
Another object is to provide such a system with improved clocking signal generating ability.
Another object is to provide such a system with an improved ability to recognize the end of a block of data.
Another object is to provide such a system with recognizable signals for framing the significant portion of each character of data.
Still another object is to provide such a system with recognizable characters framing each block of data.
These and other objects of the present invention will be apparent to those skilled in the art from the description of the invention in connection with the drawings.